1. Field of the Invention
This invention relates to a projection image display apparatus for displaying an image by projecting illumination light modulated by light valve unit of a liquid crystal panel, etc., onto a screen and in particular to an image display apparatus using light emitting diodes, semiconductor lasers, or the like as light emission sources.
2. Description of the Related Art
A projection image display apparatus illuminates light valve unit of a liquid crystal panel, etc., by an illuminating unit and projects illumination light modulated by the light valve unit onto a screen with a projection optical system. In the illuminating unit, white lamps are widely used as light sources. However, in recent years, use of light emitting diodes in place of white lamps has been examined. The light emitting diode has the advantage that it has long life, provides good energy efficiency, and has a low heating value as compared with the white lamp.
FIG. 13 is a schematic drawing of a single-plate image display apparatus using light emitting diodes as light emission sources. JP-A-10-269802 describes the single-plate image display apparatus in detail. In FIG. 13, numeral 11 denotes a light emitting diode, numeral 12 denotes a kaleidoscope for uniforming the illuminance of light emitted from the light emitting diode, numeral 23 denotes a relay lens, numeral 24 denotes a field lens, numeral 20 denotes a combining prism for combining three color light beams, numeral 22 denotes an illuminating unit comprising the light emitting diode 11 joined to the incidence end face of the kaleidoscope 12, numeral 41 denotes a translucent liquid crystal panel as light valve unit, numeral 25 denotes a projection lens, and numeral 26 denotes a screen. Suffixes R, G, and B on the reference numerals in the figure denote elements corresponding to red, green, and blue respectively.
Red illumination light, green illumination light, and blue illumination light emitted from the light emitting diodes 11R, 11G, and 11B are incident on the kaleidoscopes 12R, 12G, and 12B and are totally reflected repeatedly in the kaleidoscopes 12R, 12G, and 12B to form uniformed red light, green light, and blue light. The uniformed red light, green light, and blue light pass through the relay lenses 23R, 23G, and 23B and the field lenses 24R, 24G, and 24B and are incident on the combining prism 20 for illuminating the light valve 41. At this time, the light emitting diodes 11R, 11G, and 11B are lit repeatedly in the order of red, green, and blue. The red light, green light, and blue light are modulated to image light corresponding to a video signal by the light valve 41 and then the image light is projected onto the screen 26 through the projection lens 25 and is visually recognized as a color image by persistence of vision of a human being.
The image display apparatus using light emitting diodes, semiconductor lasers, or the like as light sources has low intensity as compared with an image display apparatus using white lamps and involves the following problem in providing high intensity:
The intensity of a light emitting diode rises in proportion to a current value, but the life of the light emitting diode is shortened as the applied current is increased. Although the intensity can also be raised by increasing the number of light emitting diodes, light emitted from a light emitting diode distant from the optical axis and is low in transmissibility in an illumination optical system and is poor in light use efficiency. If the total light emission area of the light emitting diodes exceeds a value determined by the radiation solid angle of the light emitting diode and the solid angle of illumination light allowed in an illuminated region, the light flux incident on the illuminated region becomes saturated and the light use efficiency is degraded.
Further, the light emitting diodes differ in temperature characteristic depending on the luminous color; among them, the red light emitting diode easily rises in temperature as compared with other color light emitting diodes and is degraded in output intensity because of the temperature rise and thus much current than that to any other color light emitting diode is applied, resulting in shortening the life of the red light emitting diode; this is also a problem.